This invention pertains to laminated composite shells whose surfaces are closed in the hoop direction (shapes such as a barrel-, nozzle-, or dome-like) or in both hoop and longitudinal directions (such as a pressure vessel). Composite materials technology is most advanced for essentially flat structures such as plates or shallow (slightly curved) shells and for cylindrical shells. Unidirectionally reinforced tapes or woven fabrics are used in both cases. In the construction of doubly curved shells, two main manufacturing methods are common-filament winding and laminating. Both methods generally require the utilization of mandrels, which in many circumstances need to be removed or otherwise disposed of upon completion of the manufacturing process. The disposal procedures are sometimes very cumbersome.
Filament-wound shells have a high volumetric density of fibers and implement the most efficient, isotensoid, design where the fibers are uniformly stressed. However, filament winding is a rather slow process requiring a sophisticated computer-controlled equipment to maintain a complex winding pattern with a carefully monitored fiber tension level. An inherent difficulty arises due to the high fiber overlaps and the resulting build-up of the shell thickness at the areas with a smaller radius, such as the vicinity of the shell apex.
The alternative method of construction is assembling a doubly curved shell of prepreg unidirectionally reinforced tapes or woven fabric plies on a mold or mandrel. A review of the state-of-the art manufacturing techniques of this method within the context of the resulting mechanical properties (strength, toughness) was provided by Groves (1991). The method is characterized by irregular layout patterns with extensive staggered splicing, thereby precluding an efficient utilization of the fiber strength. The method also involves a series of time-consuming and essentially manual operations. The difficulty stems from the geometric discrepancy between the doubly curved surfaces of shell structures and the geometry of flat prepreg tapes or fabrics. Although woven fabric can comply smoothly to any curvilinear surface, applying an initially flat prepreg fabric ply to a mandrel produces folds, wrinkles and air-filled voids. In their manual smoothing and rolling out, some inadvertent fiber rearrangement is unavoidable and difficult to predict or control; the process depends on such diverse factors as the shape of the surface, weave density and internal friction within the ply, interlaminar friction, viscosity of the uncured resin, and so on. The unavoidable and potentially harmful consequence of the manual smoothing is an imperfect and inconsistent prepreg lay-up geometry, in particular, fiber misalignment, waviness and microbuckling. These imperfections may be aggravated and perpetuated by the pressure applied in autoclave curing which is necessary to reduce matrix voids and to improve the volumetric density of fibers in the composite. The resulting unpredictable and randomly localized defects reduce the composite shell stiffness and strength.
The present invention is concerned with an alternative layout and manufacturing method for laminated composite shells. The objective is to obviate the above difficulties in the current state of the art and to provide some other advantages discussed below.